(NEW YORK) -- Bed bugs are on the rise again in the U.S., which means business is booming for pest control companies like Orkin.

With increased travel, both internationally and domestically, and higher bed bug resistance to existing pesticides, Orkin has seen an almost 33-percent boost in bed bug business compared to 2011.

The company has just released its rankings of U.S. cities in order of the number of bed bug treatments from January to December 2012. The "Windy City" of Chicago tops the list, followed by Detroit, Los Angeles, Denver and Cincinnati.

"This list shows that bed bugs continue to be a problem throughout the U.S.," Ron Harrison, Ph.D., Orkin entomologist and Technical Services Director said in a statement. "Based on the diversity of cities on the list, we all need to be very cautious when we travel -- whether it is business or pleasure, or to visit family, friends or vacation."

Bed bugs are about the size and color of a flat apple seed, and are found not only on mattresses and upholstery, but in suitcases, boxes, shoes, wallpaper and headboards.

Harrison stresses that it's important to be vigilant and take proper precautions wherever you are. It's a common misconception that sanitation is a factor in developing the tiny pests, Orkin says.

Here are the top 50 U.S. cities, ranked in order of the number of bed bug treatments. The number in parenthesis is the shift in ranking compared to January to December 2011: